1. Field of the Invention
The present invention relates to a stepper used for the fabrication of semiconductor elements and liquid crystal devices. More particularly, the invention relates to the improvement of the optical systems therefor.
2. Related Background Art
In a projection type aligner which is known as a stepper, an illuminating light is irradiated onto a reticle or the original of a mask and others (hereinafter, referred simply to as reticle) on which a pattern is formed for the formation of a given circuit, and the projected image of the pattern is transferred by exposure to a photoresist-coated substrate (a wafer, for example). In recent years, the pattern dimensions of the semiconductor elements have increasingly become minuter with higher densities. As a result, severer demands are imposed upon the optical conditions such as the focal depth, resolution, and transfer precision, or exposure transfer conditions.
Also, due to the production of a small quantity for a variety of different products brought about by the subdivision of functions and others of semiconductor elements, different patterns are exposed on the respective chips on one and the same wafer in some cases. In such cases, it is necessary to replace reticles while the alignment is in operation continuously. Also, along with the circuits becoming more complicated, the alignment operation should be repeated in each of the multiple layers. In such a case, the alignment patterns differ from each other for the respective layers. Each time such a different pattern is used, the severe demands on the optical conditions and others as described above must be met.
In order to meet a demand of this kind, a stepper is designed in such a manner that a variable aperture stop is provided for the pupil plane of each of the optical illumination system for converging the illuminating rays onto the reticle to irradiate it and the optical projection system to image the projected image of the reticle on the wafer, and that the aperture numbers of the optical illumination system and optical projection system are set at the optimal values to satisfy the foregoing demands by adjusting the stops to provide the appropriate amounts, respectively.
FIG. 8 is a view schematically showing the structure of a stepper provided with a prior art variable stop mechanism such as described above.
The illuminating rays from a light source (not shown) are appropriately formed and transmitted through a flyeye lens 43 as parallel beams; thus allowing the illuminating rays to be uniform. The light beams are further converged through a field lens 45 and a condenser lens 46 to irradiate with the uniform illuminance distribution the reticle 51 on which the original patterns are represented. The projected pattern image of the reticle 51 is imaged on the wafer 52 by the optical projection system 41 comprising a projection lens and others.
At the exit of the flyeye lens 43, a first variable stop (aperture stop) 44 capable of adjusting the size of the aperture as indicated by an arrow D is arranged to set the aperture number (NAi) of the optical illumination system at an appropriate value. Also, on the pupil plane positioned in the optical projection system 41, there is arranged a second variable stop (aperture stop) 42 capable of adjusting the size of the aperture as indicated by an arrow E to set the aperture number (NAp) of the optical projection system 41 at an appropriate value.
Here, the aperture number NAi of the optical illumination system is given as NAi=sin .theta..sub.I. Also, the aperture number NAp of the optical projection system 41 is given as NAp=sin .theta..sub.W. In other words, with the adjustment of the first variable stop 44, the opening angle for the illuminating beams to the reticle 51 is moderately adjusted; hence enabling an aperture number to be set desirably for the optical illumination system. Also, with the adjustment of the second variable stop 42, the aperture angle (=2.theta..sub.W) of the projecting alignment beams is moderately adjusted for the wafer 52; hence enabling the aperture number to be set desirably for the optical projection system.
On the other hand, as important factors to satisfy the required optical conditions of exposure, there are the resolving power R and the focal depth DOF as indices on resolution. These are expressed as R=k.sub.1 .times..lambda./NAp and DOF=k.sub.2 .times..lambda./(2NAp.sup.2), respectively. The k.sub.1 and k.sub.2 are constant, and the .lambda. is the wavelength of the illuminating light for exposure. The .lambda. is set constantly by the illuminating light to be employed. Therefore, in order to enhance the resolution (to make the R smaller), it is desirable to use a laser light or others having a short wavelength, at the same time making the aperture number NAp great. However, if the aperture number NAp becomes greater, the focal depth DOF becomes smaller varying inversely as the square. Hence, a problem is encountered among others that it is difficult to perform the positional adjustment between the exposing surface and the imaging plane on a wafer. On the contrary, if the aperture number NAp 6 made smaller in order to provide a greater focal depth, the value R becomes greater to lower the resolution (resolving power). Therefore, when considering the aperture number for each of the optical systems in an aligner before the execution of the pattern exposure, the requirements of the resolution and focal depth should be considered per pattern. Furthermore, various exposure conditions, and the conditions of reticle, wafer, and others should be considered in accordance with the pattern in this case so as to set an optimal value for the combination of the aperture numbers NAi and NAp of the foregoing optical illumination system and optical projection system.
FIG. 7 is a plane view illustrating the stop aperture plane to explain the distribution of the direct rays of light (zero order diffraction light) from the flyeye lens 43 of the optical illumination system arranged for the pupil plane of the foregoing optical projection system 41, namely, the plane where the variable stop 42 is installed. An inner circle 11 indicated by a dashed line represents the aperture inner periphery of a first variable stop 44 while an outer circle 12 indicated by a solid line represents the aperture inner periphery of a second variable stop 42. In the aperture periphery 11 of the first variable stop, each of the flyeye images 13 of the flyeye lens 43 is evenly distributed all over, respectively. This inner circle 11 can be expanded or contracted within a range indicated by an arrow A by increasing or decreasing the stop aperture of the first variable stop 44. Also, the outer circle 12 can be expanded or contracted within a range indicated by an arrow B by increasing or decreasing the stop aperture of the second variable stop 42.
Usually, the circle 11 of the first variable stop is set inside from the circle 12 of the second variable stop to make its .delta. value approximately 0.5 to 0.7. In other words, the stop down amount of each of the variable stops is adjusted to set the aperture numbers of the optical illumination system and optical projection system so that the direct rays of light (zero order diffraction light) transmitted through the reticle 51 of the illuminating light from the flyeye lens 43 (second light source) which has passed the first variable stop 44 can all be passed through the aperture of the second variable stop 42. Here, the value .delta. is expressed as .delta.=NAi.times..alpha./NAp where the .alpha. is the magnification of the projection exposure. In FIG. 7, it can be regarded as indicating a ratio of the image size of the stop aperture (circle 11) of the optical illumination system to the aperture (circle 12) of the optical projection system.
As describe earlier, however, if the optimal aperture numbers are individually set by the respective adjustment of the variable stops 44 and 42 each for the optical illumination system and optical projection system in accordance to each of the exposure patterns, the movable ranges A and B of the stops are overlapped in a range indicated by arrows C. Therefore, in some cases, the circle 11 of the stop on the optical illumination system side may be set larger than the circle 12 of the stop on the optical projection system side inevitably. This condition can be expressed by the following formula using the foregoing aperture numbers: EQU NAp&lt;NAi.times..alpha.
In such a state as this, a part of the beams adjusted by the stop down of the first variable stop 44 is shielded by the second variable stop 42. Therefore, not only is this unacceptable optically, but also, problems will be encountered as set forth below. In other words, whereas each blade of the stop is provided with a reflection preventive coating, the rays of light reaching the second variable stop 42 are partly reflected to scatter; thus producing a flare to cause the exposure performance and quality to be degraded. Particularly, when a laser of a far ultraviolet wavelength band, such as an excimer laser is used for an exposure light source, the coating of the blade surface is deteriorated to bring about the peeling and sublimation of coating, and other chemical changes because the direct light having a highly densified energy is converged onto the stop blade surface. Thus, the coating agent adheres to the other optical members to produce an adverse effect on the functions of such members, and other harmful effects.